Flails are one of the newest weapon types added to the game, and not many exist at the present time. Flails skill scales with brawn and senses for accuracy and skill, reflexes for speed, and brawn for bonus damage.
Good Things About Flails
Most flails are PC-, making them the only unblockable melee weapons. They also speedcap rather easily, though some seem to have speedcaps of their own making them essentially melee guns that don't use ammo.
You can make your own! Flails can be crafted on the forge from a chain, some plastic, and just about anything (cinderblocks, swords, refrigerators, live babies, etc.)
Not So Good Things About Flails
Several flails have the ability to get stuck in your opponent's face mid-battle. How is this a bad thing, you ask? Well, to continue swinging at their noggins, you'll have to pull the flail out of their body first. This may take a while if you roll poorly. Every "yank" attempt deals damage as well, and bonus damage when successfully yanked out.
Several flails, including all the improvised ones, has their attack speed behave similar to firearms. When dual wielded, their attack speeds will double instead of the typical 1.5x multiplier for dual wielding. This results in no gains in swings per seconds when dual wielded unlike all the other melee weapons.
Improvised flails' attack speed is also proportional to the weight. This results in ridiculously high reflex to speed cap, all improvised flails cap at 3.0 speed regardless of the fighting stance, the heavier flails.
Also, as they are PC-, they cannot block your opponent's attacks, forcing you to rely on your dodge and soaks to survive.
Most improvised flails require ludicrously high reflex, strength or skill to wield, with the infamous refrigerator flail taking 60-* raw to use. Basically joke weapons, use at your own discretion.